This invention relates to an improved method and apparatus for conditioning air such as warm moisture-laden air and more particularly, it relates to a novel combination for moisture removal and cooling process air using an improved indirect evaporative cooler.
Current air conditioning technology is based on compression and expansion of a gas such as chlorinated fluorocarbon or halogenated chloroflurocarbon or ammonia. The gas is compressed to a liquid state and then allowed to expand back to a vapor state. In the expansion stage of the process, heat is required to change the liquid back to a gas. However, this system has the disadvantage that the chlorinated fluorocarbons and the halogenated chlorofluorocarbons have been found to be environmentally damaging particularly to the earth's ozone layer. Ammonia results in health and safety problems when leaks develop in the system.
Vapor compression systems have another disadvantage in that they require electricity to drive compressors. At peak demand periods, the electricity required by air conditioning systems has resulted in brownouts. Thus, there is a great need for an air conditioning system that would greatly reduce the electricity demand.
Evaporative cooling technology provides an alternative to vapor compression technology. In direct evaporative cooling, water is evaporated into the air stream entering the space to be cooled. The evaporation of the water reduces the air temperature adiabatically without an enthalpy reduction. In an indirect evaporative cooling system, the primary air stream is cooled in a dry duct. An air stream is directed into an adjacent wet duct having a common wall with the dry duct. In the wet duct, water is evaporated into the air cooling the common wall and consequently the air in the dry duct. However, the evaporative cooling systems have the problem that they cannot be used in hot humid climates.
Desiccant cooling technology has been used for air conditioning wherein a desiccant material is used to remove water from air to provide a dry air stream. The dry air with is hot from an adsorption process is then cooled as closely as practical to ambient and then water is evaporated by and into the air stream to provide cooler air. This technique works well in theory; however, in practice, it has not been used very much. Conventional desiccant cooling systems have the problem that they usually require one or more heat wheels for rejecting heat from the process air stream. Heat wheels are subject to leakage. In addition, heat wheels have moving parts, add complexity and cost to the cooling system. In these applications, the air must be overdried and then rehumidified with a direct evaporative cooler.
Several attempts have been made to solve the above problems but usually with only limited success. For example, Tsimerman U.S. Pat. No. 5,050,391 discloses an apparatus and method for treating a gas in which a main gas stream is cooled and heat is transferred by convection to a liquid and a secondary gas stream across a heat transfer surface. Liquid bearing the heat evaporates into the secondary stream. A total gas stream is subjected to initial cooling along a moisture impervious surface of a heat exchange element to cause dry or sensible cooling. A secondary gas stream is diverted from the total discharge from the dry cooling stage and is directed counterflow to the total stream at the opposite surface of the heat exchange element which is an absorbent surface to cause heating and humidification of the secondary stream. The remaining cooled stream which is the primary stream is subjected to sorbent treatment to heat and dehumidify the same by flowing it through an absorbent media. The primary gas flow is then subjected to a final heat exchange treatment stage similar to the initial heat exchange treatment. Condensate may be removed from the saturated or near-saturated secondary flow. The primary flow may also be subjected to intermediate cooling treatment A portion of the initial and final dry cooling stages preferably include an adiabatic cooling zone. The secondary flow from the initial heat exchanger may be subdivided into two streams. One secondary sub-stream is dehumidified in the sorption device by an auxiliary heater and is used for regeneration of the absorbent media. However, this system discloses the use of a complicated adsorbent belt or plate and tubular heat transfer elements and therefore does not lend itself to easy commercialization.
Gunther U.S. Pat. No. 4,594,855 discloses a process and apparatus for ventilation with heat exchange wherein outdoors and indoors air streams are induced to travel in opposite directions through the adjacent passages of a plate-fin heat exchanger. A water stream is simultaneously induced to circulate through the passages occupied by the cooler of the air streams. Heat is transferred across adjacent passages, causing vaporization of a portion of the water stream into its accompanying air stream, thus keeping the cooler stream saturated with water vapor. The hotter air stream is cooled and dehumidified while the cooler one increases in temperature and humidity content.
Maisotsenko et al U.S. Pat. No. 4,977,753 discloses a method for indirect evaporative air cooling wherein the main air stream in a room is cooled by passing it along the dry duct and simultaneously passing an auxiliary air stream that has been taken from the atmosphere in a countercurrent flow along the moist duct, which is in heat-exchange interaction with the dry duct. To provide more intensified cooling, the auxiliary stream is precooled in another dry duct, which is in heat-exchange interaction with another moist duct.
Rotenberg et al U.S. Pat. No. 5,187,946 discloses an apparatus and method for indirect evaporative cooling of a fluid which includes an apparatus and method for indirect cooling of air from its ambient temperature to substantially its dew point. Ambient air is redirected through spaced openings formed along the heat exchanger plates to form a secondary air stream.
In the following patents, desiccants or adsorbents are used in conditioning air and, in particular, for lowering the humidity of the air: U.S. Pat. Nos. 2,147,248; 2,233,189; 2,266,219; 2,344,384; 3,251,402; 3,889,742; 4,113,004; 4,121,432; 4,719,761; 4,723,417; and 4,729,774.
Kaplan U.S. Pat. No. 5,170,633 describes a method and apparatus for conditioning air utilizing a desiccant based air conditioning system requiring substantially less regeneration energy than typical systems. This regeneration energy reduction is accomplished through the use of two separate desiccant devices and an indirect evaporative cooler having both a wet and dry side for air flow-through. In the first desiccant device regeneration air is first passed through the wet side of the indirect evaporative cooler wherein it is humidified and heated. This air is then dehumidified by passing it through the second desiccant device which is operated at a high moisture content. This results in a substantial amount of moisture being adsorbed from the first regeneration air stream causing a substantial air temperature increase and thereby, reducing the auxiliary heat required. The second desiccant device may be regenerated with ambient air.
Meckler U.S. Pat. No. 3,488,971 discloses air for comfort conditioning which is circulated through a chemical dehumidifier and then is forwarded to a space to be conditioned. Optional sensible cooling coils before and after chemical dehumidification are provided. Most of the lighting heat and most of the space load are transferred to relatively high temperature water, and from the water to the chemical dehumidifier for regeneration of the desiccant therein. A thermally activated heat pump, which can be of the thermoelectric type, is used to transfer heat to the high temperature water.
In spite of these disclosures, there is still a great need for a compact, economical air conditioning unit that functions in warm, moisture-laden air, e.g., outdoor air, or in warm dry air and which does not require large amounts of electricity in order to function. The present invention provides such an air conditioning system utilizing a novel combination of desiccant systems and evaporative cooling systems.